Possible phase transition of water in polymer-water systems has been understood (assumed) to be only either freezing/melting or nothing, without relation to whether the polymer was Soluble in water or whether its water content was high or low. This general understanding of water structure has been structured on the basis of calorimetric analyses Such as differential scanning calorimetry, DSC. DSC is one of the most frequently used methods to analyze the water Structure in polymer-water systems because the data obtained are relatively easy to interpret. This easiness of interpretation, however, can be accepted only if based on the understanding stated above. Unexpectedly, results of the infrared spectroscopic analysis presented here completely denied the general understanding and provided definite evidence of the existence of all phase transitions among the three states of water in a polymer solid. Furthermore, the impossibility of detection of condensation, deposition, sublimation, and vaporization by calorimetric analysis was revealed.